Correct option (d) 17.2s
Explanation:
Given : u = 0, t = 5 sec, v = 108 km/hr = 30m/s
By eqn of motion
v = u + at
or a = c/t = 30/5 = 6m/s2 [u = 0]
S1 = 1/2at2
= 1/2 x 6 x 52 = 75
Distance travelled in first 5 sec is 75m.
Distance travelled with uniform speed of 30m/s is S2
395 = S1 + S2 + S3
395 = 75 + S2 + 45
∴ S2 = 395 – 120 = 275 m
Time taken to travel 275 m = 275/30 = 9.2sec
For retarding motion, we have
02 – 302 = 2(– a) × 45
We get, a = 10 m/s2
Now by, S = ut + 1/2at2
45 = 30t + 1/2(-10)t2
45 = 30t - 5t2
on solving we get, t = 3 sec
Total time taken = 5 + 9.2 + 3
= 17.2 sec.